In recent years, along with the increasing information capacity, WDM optical transmission systems have been playing an important role in reducing the cost of systems. In a WDM optical transmission system, WDM light, which is generated by multiplexing a plurality of signal lights of different wavelengths, is transmitted in a multi-relay manner using optical amplifiers. A WDM optical transmission system may be further reduced in cost by using a configuration of nodes that is suitable for the intended application.
FIG. 1 illustrates an exemplary configuration of nodes in a typical WDM optical transmission system 100. The WDM optical transmission system 100 includes a plurality of nodes, that is, terminal stations 110 and 150, an optical add/drop multiplexing (OADM) station 130, and relay stations 120 and 140.
The terminal stations 110 and 150 are end nodes coupled to the two ends (transmission end and reception end) of a transmission path L along which a signal light is transmitted in a single direction. The terminal station 110 on the transmission side multiplexes, using a multiplexer 112, signal lights output from optical transmitters (TX) 111 corresponding to used wavelengths of a WDM light so as to generate a WDM light, amplifies the WDM light using an optical amplifier 113, and transmits the amplified WDM light along the transmission path L.
The terminal station 110 on the transmission side serves as a starting point of a propagation path (optical path) of signal lights corresponding to respective wavelengths. The terminal station 150 on the reception side amplifies, using an optical amplifier 151, the WDM light transmitted along the transmission path L, demultiplexes the WDM light into signal lights of respective wavelengths using a demultiplexer 152, and performs reception processing on the individual signal lights using corresponding optical receivers (RX) 153. The terminal station 150 on the reception side serves as an end point of the optical path.
The OADM station 130 is a node that is positioned along the transmission path L and that has an OADM function. The OADM station 130 amplifies, using a preamplifier (PreAMP) 131, the WDM light received through the transmission path L, demultiplexes the WDM light into signal lights of respective wavelengths using a demultiplexer 132, and supplies one or more signal lights (DROP) that are to be branched off at this node to one or more corresponding receivers (RX) 133.
In addition, the OADM station 130 multiplexes, using a multiplexer 135, the remaining signal lights (THRU) obtained through demultiplexing performed by the demultiplexer 132 and one or more signal lights (ADD) output from one or more corresponding transmitters (TX) 134 so as to generate a WDM light, amplifies the WDM light using a post amplifier (PostAMP) 136, and transmits the amplified WDM light along the transmission path L. The OADM station 130 may selectively serve as the starting or end point of the optical path, or a relay point for relaying a WDM light to the next node, depending on the individual wavelengths of the WDM light.
The relay stations 120 and 140 are optical relay nodes positioned along the transmission path L, and amplify a WDM light the optical level of which has been decreased by a loss characteristic of the transmission path L using inline amplifiers (In Line Amplifiers (ILAs)) 121 and 141, so as to compensate for the loss of the transmission path L. The relay stations 120 and 140 serve as relay points of the optical path corresponding to the individual wavelengths.
In WDM optical transmission systems of the above-described type, erbium-doped fiber amplifiers (EDFAs), which efficiently amplify a WDM light in a collective manner, are widely used as optical amplifiers provided in individual nodes. Regarding the EDFA, it is known that a physical phenomenon called polarization hole burning (PHB) occurs when a high-power signal light is input to an erbium-doped fiber (EDF). In PHB, the gain of a light containing polarized waves parallel to the polarization direction of the signal light is suppressed.
In the WDM optical transmission system, the number of wavelengths included in a WDM light may be arbitrarily set or controlled. When a signal light of a small number of wavelengths (for example, one wavelength) is transmitted, the degree of polarization of the transmitted light is higher than when a signal light of a large number of wavelengths is transmitted. If such signal light having a high degree of polarization is input to an EDFA, an optical signal to noise ratio (OSNR) of an amplified light is significantly degraded due to the occurrence of the above-described PHB.
That is, the gain with respect to a signal light having a high degree of polarization, and the gain with respect to amplified spontaneous emission (ASE) containing polarized waves parallel to the polarization direction of the signal light are suppressed by PHB. However, the gain with respect to ASE containing polarized waves perpendicular to the polarization direction of the signal light is relatively increased because the gain is not affected by PHB. As a result, compared to a case where PHB does not occur, the ratio of ASE containing polarized waves perpendicular to the polarization direction of signal light increases, and the OSNR is degraded.
Regarding the related art for suppressing the above-described degradation of the OSNR caused by PHB, Japanese Laid-open Patent Publications No. H07-22683 and No. H08-316910 disclose examples of the related art of changing the polarization state of a signal light input to an EDFA using modulation or the like and decreasing the degree of polarization of the signal light, so as to suppress the occurrence of PHB.
In addition, Japanese Laid-open Patent Publication No. 2009-290593 discloses an example of the related art of supplying, together with a signal light, ASE in a wavelength band corresponding to an unused wavelength adjacent to a used wavelength contained in ASE generated by a preamplifier of an OADM station to a post amplifier (EDFA), thereby decreasing the degree of polarization of a light input to the post amplifier so as to suppress the occurrence of PHB and to suppress the degradation of the OSNR.
The related art utilizing ASE does not directly change the polarization state of a signal light, but indirectly decreases the degree of polarization of the signal light by using non-polarized ASE at a neighboring unused wavelength, and thus may be realized with a simple configuration and has a small influence on a signal light.
However, it is basically difficult to apply the above-described examples of the related art utilizing ASE to a node other than an OADM station. That is, the suppression of PHB according to the related art is realized by decreasing the degree of polarization of light input to a downstream optical amplifier (EDFA) by using non-polarized ASE generated through amplification performed upstream.
Thus, in a node without an upstream source of ASE, such as the terminal station 110 on the transmission side in the WDM optical transmission system 100 illustrated in FIG. 1, for example, it is difficult to suppress the occurrence of PHB by applying the related art. Therefore, if a signal light having a high degree of polarization is input to an optical amplifier (EDFA) in a terminal station on the transmission side, the OSNR of the signal light output from the optical amplifier is inevitably degraded.
In particular, when many relay stations are provided in a section between the terminal station on the transmission side and an OADM station, PHB that occurs in the terminal station on the transmission side has an influence on the entire section, and thus the OSNR of the signal light is significantly degraded. That is, if the related art is applied to the node configuration illustrated in FIG. 1, the occurrence of PHB may be suppressed in the OADM station 130 and the nodes downstream of the OADM station 130, but degradation of the OSNR of a signal light caused by PHB may not be suppressed over the entire WDM optical transmission system 100.